MongoDB storage provider for Jinaga distributor

[michaellperry]

Create an implementation of StorageProvider backed by MongoDB, using the Node MongoDB Native driver. The new class should pass the Memory tests. This should be suitable for use in the distributor.

Refactor the storage provider tests so that they can be executed on any StorageProvider instance. As new features are added to the storage provider, or new tests are devised for existing features, all storage providers should be subject to the same suite of tests.